The present invention relates to the drying of plyboard and the like in continuous-operation drying installations wherein the plyboard to be dried continuously travels through a long drier consisting of a succession of drying zones. In each drying zone, air is circulated in the direction transverse to the transport direction of the goods through the drier, the air in each drying zone being driven by fans to sweep over a heating battery and circulating about the goods passing through the respective zone, the air being blown onto the goods to be dried in that zone through nozzles. In particular, the present invention relates to monitoring and controlling the operation of such conventional drying installations. The monitoring and controlling of such installations can become very problematic when, as is typical, both the degree of loading of the drier and also the initial moisture content of the goods to be dried fluctuate markedly. The degree of loading of the drier is typically expressed by the amount of surface area of the goods to be dried per unit length.
For example, when passing plyboard through such a conventional drier, the degree of loading (amount of surface area at which drying is to be effected) and the initial moisture content of the plyboard can vary very considerably. If the product emerging at the output end of the succession of drying zones is to nevertheless be of uniform residual moisture content and general quality, then it is necessary that the operator, in quick response to such fluctuations in the degree of loading and initial moisture content, adjust the transport speed at which the goods are being transported through the drier and/or adjust the drying parameters, e.g., temperature of the hot air used for drying, moisture content of the hot air employed, etc.
Very often, for the sake of simplicity, the average temperature conditions of the hot drying air in the succession of drying zones is kept more or less constant, and then to vary the drying effectiveness in response to fluctuating drying demand, the operator simply increases and decreases the transport speed using a control lever, or the like, to thereby vary the amount of time the goods to be dried dwell in the successive zones of the drier. In order to proceed on this basis, frequent evaluation of the residual moisture content of the emerging dried goods is necessary. Accordingly, the operator's attempt to match the drying action afforded to the drying demand is very approximate and coarse. Furthermore, attempting to match the prevailing drying action to the prevailing drying demand in this simple way, i.e., by control of dwell time only, can be problematic in highly automated installations; for example, if the long drier is connected to a long production line, and the goods are fed to the drier automatically and automatically removed from the drier, adjustment of dwell time in this way can conflict with the speeds of operation and productivity of processing units located upstream and downstream of the drier.
It is known to positively measure the initial moisture content and degree of loading (surface area) of the goods to be dried before they actually enter the drier, and to adjust the average drying temperatures of the hot air in the successive drying zones accordingly, in an attempt to match drying action to drying demand. However, positively measuring these two quantities can be very troublesome and problematic, for example requiring the use of a kiln-drying test, and despite the troublesomeness is not always sufficiently reliable.
To improve the quality of the dried product emerging from the drier, it is known not to adjust the temperature of the hot drying air incident upon the dried goods, but instead to attempt to meet drying demand by measurement and negative-feedback control of the temperature of the hot drying air which has already contacted and is leaving the dried goods. This decreases the problematic tendency towards insufficient drying action typically encountered when the degree of loading (i.e., surface area to be dried) is high. It is also known to try to improve quality by using negative-feedback control to maintain constant the moisture content of the recirculated hot drying air at the point where the hot drying air has already contacted and is leaving the goods.
These various techniques all have characteristic shortcomings.